The use of glass fibres for the transmission of telecommunications signals at wavelengths between 800 and 1600 nm is well established. It has also been proposed to utilise glass fibre as an active element, e.g. as a laser or amplifier. This requires the incorporation of dopants into the glass of which the fibre is made. For example, the incorporation of rare earths such as neodymium provides lasing properties.
Glass fibre is made by first preparing a precursor in the form of a thick rod having suitable composition gradients arranged in its cross section. The precursor is drawn into the fibre. One of the standard methods of making the precursor is known as MCVD or modified chemical vapour deposition. MCVD comprises passing suitable reactants, e.g. SiCl.sub.4 and O.sub.2, through the bore of a substrate tube, a small length of which is heated to reaction temperature. The heated segment moves along the length of the tube thereby depositing a thin layer of new glass on the inner wall. Many layers, e.g. 20 to 30, are usually deposited. The composition of each layer is individually controlled so that the cross-sectional composition of the ultimate fibre is also controlled. When enough layers have been deposited the tube is collapsed to a solid rod which is drawn into fibre.
MCVD is usually carried out with reactants which are volatile at room temperature, e.g. SiCl.sub.4 (to provide the SiO.sub.2 which is the major component of the ultimate fibre) and GeCl.sub.4 (which provides the GeO.sub.2 to adjust the refractive index). The rare earths are difficult to utilise in this process because they do not form suitable volatile compounds. A paper published by Poole, Payne and Fermann of Southampton University in "Electronics Letters" 15 Aug. 1985, Vol. 21, No. 17 describes a process in which Nd is introduced into MCVD by heating NdCl.sub.3 to about 1000.degree. C. Anhydrous NdCl.sub.3 is deposited near the inlet of the substrate tube (but outside the deposition zone). When Nd is required, the deposit is heated to a suitable temperature, e.g. 1000.degree. C., using a second burner (the first burner heats the moving segment of the deposition zone).
It has been shown that this technique is effective for incorporating Nd into an optical fibre but the process is too variable and it lacks sufficient control. This invention relates to a new source which facilitates control and thereby provides more consistent products.
The new source takes the form of an inert, solid, porous sponge which is impregnated with a metal compound, preferably involatile at temperatures below 100.degree. C., e.g. a salt of a rare earth metal such as a neodymium or erbium salt. In use, the sponge is placed in the substrate tube upstream of the deposition zone and heated to volatilise the impregnant into the reactant stream.
The incorporation of suitable levels of dopants into the tube is achieved with low concentrations of the metal compound in the reactant gas stream of the MCVD, e.g. concentrations as low as 0.01% molar. Since the source can be heated to 1000.degree. C. or even higher, e.g. the melting point of silica, many metal compounds can be used as the source. For example, there are many metal salts and other compounds giving partial vapour pressures of approximately 0.1 or greater Torr at 1000.degree. C. Thus the chlorides are particularly suitable as impregnants because they are usually the most volatile salt and other chlorides, e.g. SiCl.sub.4 and GeCl.sub.4, are present in the system.
The most suitable configuration for the sponge is a tube, preferably with an outer impervious layer and an inner porous layer so that volatilised impregnant is preferentially directed into the bore. The tubular configuration allows the reaction gas to pass through the bore and the impregnant is volatilised from the porous layer into the gas stream flowing through the bore. This arrangement has minimal effect on the reactant gas used for MCVD.
A sponge must be impregnated before it can be used. The impregnation can be conveniently achieved by soaking the sponge in a low surface tension solution of the impregnant, until the solution percolates throughout all the pores. After the saturation the sponge is removed from the solution and dried by heating, e.g. at 120.degree. C. to 500.degree., preferably in an atmosphere of chlorine diluted with an inert gas such as He. The chlorine assists drying and, when the impregnant is a chloride, the chlorine may also reduce the amount of decomposition.
Sponges of this nature are conveniently prepared by MCVD deposition of porous pure SiO.sub.2 on a suitable substrate tube. After impregnation a long tube can be cut into many, e.g. 30 to 60, segments. This produces the individual sources which are used once only.
The process of the invention can be used to incorporate a plurality of rare earths. This is preferably done by employing a plurality of sponges, each impregnated with a salt of only one rare earth. The sponge can be impegnated with a mixture of several salts but this limits the control of the ratios of the various additives.
It should be realised that the amount of impregnant in the sponge does not exert a major control on the concentration thereof passed into the process (because the concentration of a solid is constant provided it is present). Thus the depletion of the sponge during use does not cause unacceptable variations of concentration. Control is achieved by using higher temperatures to volatilise the impregnant at a higher rate. The length of a sponge may also have an effect because a larger surface tends to evaporate impregnant at a higher rate.